It is well-known that when an amplifying element such as a transistor is employed in an amplifier circuit, biasing is necessary to establish an appropriate operating point for the amplifying element. With no excitation applied to the element, the operating point is defined in an active region of the output characteristics thereof. This operating point is frequently referred to as the "Quiescent (Q) Point" or the "Direct Current (DC) Operating Point".
To provide linear amplification for a radio-frequency (RF) input signal for example, a class A amplifier is normally used, wherein the DC operating point of an amplifying element is placed far enough from the cutoff and saturation regions in its output characteristics. This ensures that the anticipated input signal excursions to either side of the operating point do not cause the amplifying element to cut off the signal or become saturated, thus avoiding distortions in the amplifier output. However, a major drawback of the class A amplifier is its low power efficiency due to a relatively high DC input power required by the amplifier, with respect to its output signal power. In addition, as the input power to the class A amplifier is reduced or backed off from the maximum input power, the amount of current drawn by the amplifier quickly approaches that of the high quiescent current, without significantly reducing its power consumption. In some cases, the current draw can actually increase as the input power is backed off, resulting in no reduction in power consumption.
A class B amplifier has its amplifying element biased to cut off a half-cycle of an input signal, with an output current flow only during the positive half-cycle of the signal. As a result, the amplifier output is significantly distorted. Thus, a class B amplifier is unsuitable for a linear operation. However, with respect to a class A amplifier, a class B amplifier affords a higher power efficiency as the DC input power to the amplifier is relatively low.
A class A-B amplifier, on the other hand, has its amplifying element biased such that the output current flows for more than half of the cycle of the input signal. As a result, a class A-B amplifier behaves like a hybrid between class A and class B amplifiers. Thus, a class A-B amplifier causes a lower distortion than a class B amplifier (but a higher distortion than a class A amplifier) to an input signal at a high drive level. At the same time, the class A-B amplifier realizes a higher reduction of power consumption than a class A amplifier (but a lower reduction than a class B amplifier) during an input power backoff.
An amplifier circuit can also have its amplifying element biased dynamically. One such amplifier circuit is disclosed in U.S. Pat. No. 5,136,257 issued to Reading. The disclosed amplifier circuit is an RF power amplifier having three amplifying elements. The amplifier has a control circuit providing a feedback signal to a biasing circuit for dynamically varying the operating points of the amplifying elements. The feedback signal represents the difference between a level control voltage and the amplifier output. However, like other circuits involving a feedback, the Reading amplifier is susceptible to instability of the feedback loop, and is required to have a considerable tolerance of feedback accuracy. In addition, the Reading amplifier does not provide linear amplification over a broad output range.
The above prior art amplifier circuits are not desirable for use in many digital communications systems including digital cellular telephones. For example, a digital cellular telephone using a standard code division multiple access (CDMA) scheme is required to provide an output power range of more than 74 dB. Furthermore, the digital modulation pursuant to the CDMA scheme calls for linear amplification. For details on the CDMA scheme, one may refer to: J. White, "What is CDMA?", Applied Microwave & Wireless, Fall 1993, pp. 5, 6 & 8. Moreover, even though a cellular telephone usually operates at a low output power level (i.e., a large backoff from the maximum required output), the power efficiency has to be high because the cellular telephone is normally powered by a battery having a limited capacity, which accordingly affords a limited transmission time.
In view of the foregoing, there exists a need to improve the prior art amplifier circuits for use in communications systems, in particular, the digital cellular telephones, to achieve linear amplification over a broad output power range and, at the same time, a high power efficiency.